1. Field of the Invention
The present invention is directed to a method and apparatus for sequential deposition of thin-film coatings successively onto a web in the presence of a vacuum, and in one aspect, to an improved apparatus having a plurality of deposition chambers arranged within a single high vacuum chamber for the application of different thin-film coatings serially.
2. Description of the Prior Art
A variety of applications is known for thin-film coating application wherein said coating comprises at least two layers of different composition. In particular, in the manufacture of photovoltaic devices, the need exists for an efficient and economical generation of multi-layer thin-film coatings. This need is increased when the electrical and photoresponsive characteristics of the individual layers must be controlled.
U.S. Pat. No. 3,294,670, Charschan et al., discloses a continuously operating vacuum processing apparatus including a plurality of interconnected, open-ended chambers, with individual chambers surrounding aperture portions of a channel and containing processing atmospheres to which material in the channel is exposed. In this device means are connected to the multiple chambers for maintaining a predetermined evacuated pressure in the chambers. The apparatus is said to be especially useful for depositing thin films of tantalum, as utilized in electronic circuit fabrication, wherein controlled oxidation as provided by the deposition apparatus leads to reproducible electrical properties.
Foehring et al., in U.S. Pat. No. 3,805,736, disclose an apparatus for carrying out mass transfer reactions in a reaction chamber utilizing laminar flow to provide diffusion limited transport and isolation between process steps. The deposition apparatus includes means defining a vapor transport zone within a deposition chamber comprising a first vapor diffuser means extending longitudinally across said vapor transport zone as adapted to dispense a first gaseous phase material in laminary flow which is substantially parallel to a substrate surface and perpendicular to the path of substrate travel, with exhaust means disposed in opposed relationship and at least one additional adjacent vapor diffuser means. The vapor transport zones are structurally in fully open communication with one another, and process wise, substantially serially isolated as a result of laminar flow.
Wakefield, in U.S. Pat. No. 3,965,163, discloses a method of forming semiconductor grade silicon from lower grade silicon wherein a p-type layer is formed over an n-type layer via diffusion into the n-type layer in a second reaction chamber.
Small et al., in U.S. Pat. No. 4,015,558, disclose a multi-layer vapor deposition apparatus which discloses a plurality of serially arranged coating chambers with valve means disposed at opposite ends of each of the associated exit and entrance chambers for sealing the coating chambers and interrupting communication with the coating chamber along with individual means for creating a vacuum condition in each of the coating chambers.
Matatsgu Izu et al., in U.S. Pat. No. 4,400,409, disclose a method of making a photovoltaic panel comprising forming a roll of a web of a flexible substrate material with one or more electrode forming regions thereon, unrolling said substrate roll substantially continuously into a partially evacuated space including at least one silicon depositing region therein where there are deposited over at least some of said one or more electrode forming regions at least two thin flexible silicon films of opposite conductivity (P and N) type, one or more of said films forming a photovoltaic depletion region, said substrate formed in a substantially continuous web, and each of said silicon films is deposited in a separate glow discharge region past which said web is moved to form a substantially continuous deposition process. According to Izu et al, each of the deposition chambers, when a plurality of same are used, is connected by an isolation passageway and the exhaust from each chamber should be sufficient to isolate each of the chambers; however, an inert carrier gas can be bled into each passageway to sweep the passageway clear of any gases from the chamber on either side of the passageway. At column 13, lines 14-17, it is indicated that "It also is feasible that all of the chamber deposition areas be enclosed in a single chamber isolated one from another", however, there are no teachings as to how this might be accomplished or what benefits might accrue.
Cannella et al., in U.S. Pat. No. 4,438,723, disclose a multiple chamber deposition system and isolation means including a slot interconnecting first and second chambers and means for establishing a gas flow from the slot into the first chamber at a rate sufficient to maintain at least 10.sup.4 ratio of the concentration of an element in the first chamber to an element in the second chamber.
In contrast to the aforereferenced background art, the instant invention comprises a unitary vacuum chamber having contained therein a plurality of discrete deposition chambers including conductance limiting slits with a single vacuum pump means providing adequately low pressure for simultaneous deposition of material in each of the chambers without detectable diffusion of one process gas from one chamber into other chambers.
It is, therefore, an object of the instant invention to provide for a multiple coating vacuum deposition system which is less complex than prior art vacuum deposition systems and thus subject to less downtime.
It is, additionally, an object of the present invention to provide for a multiple chamber vacuum deposition system which is, as a function in reduced complexity, more economical to construct and operate.
It is, further, an object of the present invention to provide for a multiple chamber vacuum deposition system wherein superior levels of isolation between deposition chambers are obtained.